Participants will undergo various procedures, including physical and neurologic examinations, blood and urine tests, and magnetic resonance imaging (MRI) to determine the extent and severity of disease. MRI scanning uses a strong magnetic field and radio waves to show structural and chemical changes in the brain. During the procedure, the patient lies on a table in a narrow cylinder containing a magnetic field. He or she can speak with a staff member via an intercom system at all times during the procedure. Patients will also have a lumbar puncture (spinal tap) to examine the cerebrospinal fluid (CSF), which bathes the brain and spinal cord. To obtain the fluid, a local anesthetic is administered and a needle is inserted in the space between the bones in the lower back where the CSF circulates below the spinal cord. A small amount of fluid is collected through the needle. Although spinal fluid will not be examined regularly, this test may be requested during some clinic visits.